For lighting units for liquid crystal display screens, widely used is a edge light system in which a cold cathode-ray tube serving as the source of light is disposed at the edge of a light guide plate (JP-A 62104/1988). In the lighting system of the type, a reflector is disposed around the cold cathode-ray tube for increasing the lighting efficiency therein, and a reflecting sheet is disposed below the light guide plate for efficiently reflecting the light having been diffused through the light guide plate toward a liquid crystal display screen. In the system, these have the function of reducing the loss of light from the cold cathode-ray tube and increasing the brightness of the liquid crystal display screen. However, the edge light system could not increase the brightness of recent wide screens of liquid crystal TVs, for which, therefore, a direct back light system is being employed. In the direct back light system, cold cathode-ray tubes are aligned in parallel below a liquid crystal display screen, and they are above a reflecting sheet in parallel with each other. The reflecting sheet may be flat or may be semi-circularly shaped to partly cover each cold cathode-ray tube.
For use in the surface light source for such liquid crystal display screens, the reflector and the reflecting sheet (these are generically referred to as a reflecting structure for surface light source) are required to have high reflectivity, for which generally used are films with white dye or pigment added thereto or films with fine voids therein either alone or as laminated with any of metal or plastic plates. The films with fine voids therein are widely used as their ability to improve screen brightness and uniformity is good. They are disclosed in, for example, JP-A 322153/1994 and 118433/1995.
The recent expansion of the use of liquid crystal display screens is remarkable. For example, they are being widely used not only in conventional notebook-size personal computers but also in desk-top personal computers, liquid-crystal TVs, mobile telephone displays, various game computers, etc. With the expansion of their use, the screens are desired to have increased brightness and increased fineness, for which the power of the lighting source is increased and the number of the light source lamps is increased. In addition, for satisfying the requirement of long-term lighting of wide screens of liquid crystal TVs, they are required to have more increased brightness and durability. In particular, when the screens are lighted by a direct back light system, they shall directly receive the light from the light source. For these, the durability of the reflecting sheet is required to be higher. However, reflectors and reflecting sheets of conventional film are problematic in that the film is aged and yellowed when used for a long time and therefore its reflectivity is lowered with time and the screen brightness is after all lowered.
The present invention is to solve the problems noted above, and its object is to provide a reflecting structure for surface light source of which the brightness lowers little with time even when used for a long time and which ensures high-quality images for a long time.